Cellular telephone systems provide radio communication between a network of geographically separated, stationary base stations and mobile radio-telephones, e.g., mobile units installed in automobiles. Each base station defines a particular cell of the cellular system, and has at least one transceiver with the capability of calling to and receiving calls from mobile units in the corresponding cell. Such a cellular telephone system is described in the Bell System Technical Journal, January 1979, entitled, "The Cellular Concept," page 15, et seq.
Cellular telephone systems are subject typically to governmental regulations and approvals, and must comply with standards, such as, for example, those set forth for the United States of America in EIA RS-553 by Electronic Industries Association, Washington, D.C., U.S.A.
As set forth in those standards, a central authority assigns a unique digital code for identifying each cellular telephone system, called the system identification code ("SID"). When a mobile unit subscribes or registers for service on a particular cellular system, that system is called its "home" system.
A number assignment module ("NAM") on-board the mobile telephone stores the SID that identifies the mobile unit's home system, called the "SID.sub.p," in a non-volatile, i.e., permanent, memory. The NAM also stores a mobile identification number (MIN), which represents the mobile unit's 10-digit telephone number, in association with the SID.sub.p.
Typically, a cellular telephone system allocates to each base station a plurality of radio channels to carry voice signals (called "voice channels"), and at least one control channel. In turn, the base station assigns the allocated voice channels to mobile units for use during voice telephone calls.
The assignments of the voice channels are made in response to requests from the mobile units to the base stations for communication service. Typically, the mobile units request service from the home-system base stations from which the mobile units receive the strongest control signals over the control channels.
More specifically, a mobile unit typically "scans" for service, i.e., the mobile unit scans the strengths of control signals on pre-determined control channels, and, for instance, selects the channel having the strongest signal for further processing. The mobile unit then examines the selected control signal to determine if it originated from a base station belonging to the mobile unit's home system.
Typically, this is accomplished by the mobile unit extracting and examining the data contained in an overhead message train ("OMT") contained within the selected control signal. Each OMT contains system information including the system identification code ("SID.sub.r ") of the base station from which it was broadcasted. The mobile unit compares the extracted SID.sub.r with the mobile unit's stored SID.sub.p. If the two codes match, the mobile unit sends a request over the corresponding control channel to the base station for the assignment of a voice channel, and two-way communication can ensue over the assigned voice channel.
On the other hand, if the two codes do not match, then the mobile unit must wait until an acceptable control signal is received, which may require that the mobile unit move to another cell, or that the mobile unit resort to roaming to establish telephone communication.
In roaming, the mobile unit uses the services of a "foreign" cellular system, i.e., a system that is not its home system, and, unfortunately, the mobile unit incurs roaming fees for such services. Moreover, roaming is only available where the home system has a roaming contract with the foreign system. Thus, in certain areas outside the coverage area of the home system, or where communication with home-system base stations proves impossible or inadequate due to signal propagation conditions or other reasons, and where roaming is not available because there is no inter-system roaming agreement between the home and local cellular systems, cellular services are not available. The unavailability of service in those locales can present serious drawbacks to telephone users.
It has been proposed to provide mobile radio-telephones with multiple NAM's, each storing a MIN and an associated SID.sub.p. This permits the mobile unit to subscribe for service on several cellular systems. In essence, the mobile unit has many different phone numbers, and can have many different home systems, each associated with one of the phone numbers. In all known approaches, the mobile unit can have, however, only a single NAM operative at a time.
For instance, U.S. Pat. No. 4,734,928, suggests the use of multiple NAM's, which the telephone user manually switches into operation, e.g., whenever the user desires service from a different home system, or desires to take advantage of inter-system roaming agreements of a different home system.
Consequently, a mobile unit equipped with multiple NAM's can provide the user with service over a broader geographic range of use. In addition, the user can avoid paying roaming charges by subscribing with a foreign system as a second home system. This may permit many users to almost always be within one of its home systems. Furthermore, the user can take advantage of billing-rate differences offered by its various home systems so as to reduce telephone call charges.
Unfortunately, manually switching from one to another of the NAM's in a mobile unit requires that the user maintain a good working knowledge of the cellular systems, including their geographic boundaries. For at least that reason, many users might find manual switching of NAM's too complicated or bothersome.